1. Field of Endeavor
Example embodiments generally relate to methods of fabricating organic thin film transistors, and more particularly, to methods of fabricating organic thin film transistors comprising a substrate, a gate electrode, a gate insulating film, metal oxide source/drain electrodes, and an organic semiconductor layer, in which the surfaces of the metal oxide source/drain electrodes are treated with a self-assembled monolayer (SAM) compound containing a live ion.
2. Description of Related Art
After the development of polyacetylene, a conjugated organic polymer having semiconductor properties, organic semiconductors have received attention as electronic materials as a result of certain of the advantages associated with organic semiconductor material over conventional inorganic materials. These advantages include, for example, a variety of synthesis methods, easy formability into fibers or films, flexibility, conductivity, and relatively low preparation costs. Organic semiconductor materials have, therefore, been of interest and a subject of study in the field of functional electronic devices and optical devices.
With respect to electronic devices incorporating one or more conductive polymers, research into organic thin film transistors including an active layer formed of organic material began in about 1980, with research still ongoing throughout the world. Organic thin film transistor have a structure generally corresponding to conventional inorganic semiconductors, for example silicon (Si) thin film transistors but in which at least one of the conventional semiconductor regions have been replaced with a suitable organic material.
Compared to conventional silicon thin film transistors, organic thin film transistors can simplify the fabrication process by permitting a semiconductor layer to be formed using a printing process at atmospheric pressure, thereby avoiding the need for chemical vapor deposition (CVD) processes using plasma. Indeed, the process of fabricating an organic semiconductor pattern may be carried out using a roll-to-roll process on a plastic substrate, if desired, which may, in turn, decrease the overall cost and/or time required for fabricating the transistor. Accordingly, organic thin film transistors are expected to be utilized in a variety of applications including, for example, driving devices for use with active displays, smart cards, plastic chips for inventory tags, for example, radio frequency identification chips (RFID).
As known to those skilled in the art, however, organic thin film transistors are also associated with certain drawbacks including, for example, lower charge mobilities, higher operating voltages and/or higher threshold voltages, when compared with conventional silicon thin film transistors. It has been reported that the charge mobility within the organic semiconductor may be increased to a level of 0.6 cm2·V−1·sec−1 using pentacene, thus considerably increasing the probability of actually realizing an organic thin film transistor. However, even the improvements reflected in these reports, the charge mobility is still generally considered unsatisfactory. Indeed, in some instances operating voltages in excess of 100 V or more and threshold voltages on the order of at least 50 times the typical threshold voltage for a corresponding silicon thin film transistor are required for device operation.
In those instances in which the organic thin film transistor is configured as a bottom contact type or top gate type organic thin film transistor, the organic semiconductor material(s), for example, pentacene, tends to grow relatively less on the source/drain electrodes when compared with corresponding growth on the gate insulating film and, as a result, tends to exhibit a work function higher than that of the metal(s) forming the source/drain electrodes. Accordingly, an undesirable Schottky barrier tends to be formed between the source/drain electrodes and the organic semiconductor layer, thereby tending to decrease the charge mobility of the organic thin film transistor.
In this regard, a method of fabricating an organic thin film transistor has been disclosed in which the exposed surfaces of source/drain electrodes are treated with an SAM compound containing a thiol functional group before depositing an organic semiconductor layer, thereby attempting to increase the electric performance of the organic thin film transistor, in particular, its charge mobility. However, because the compound mentioned above binds only to the surface of metal, for example, gold (Au), but does not bind to metal oxide, for example, ITO (Indium Tin Oxide), the organic thin film transistor including the source/drain electrodes made of metal oxide and the organic semiconductor layer cannot generally not reach a desired degree or magnitude of charge mobility.
In addition, another method of fabricating an organic thin film transistor comprises treating the surface of metal oxide source/drain electrodes with an SAM compound containing a sulfonic acid group, thereby changing the hydrophobic properties of the source/drain electrodes and increasing the charge mobility. In this case, the SAM compound acts to reduce or eliminate the formation of Schottky barriers between the source/drain electrodes and the organic semiconductor layers. The SAM compound also tends to decrease contact resistance as a result of physical adsorption or chemical bonding to metal oxide, thereby tending to increase charge mobility. However, in the organic insulator formed through a surface crosslinking process, metal ions, for example, a hydrogen ion (H+), are mainly generated as by-products of the crosslinking polymerization resulting from the properties of organic insulator material itself, thereby undesirably causing increased hysteresis, a condition that negatively affects the reliability with regard to the actual operation of the device.